University of Vermont
Department of Plant and Soil Science

Winter News Article

Whether or not a plant is hardy in the cold north isn’t as simple as
just looking at a hardiness map. There are several factors
affecting hardiness you should understand in order to know better
which plants to grow, where to place them, and why some may have
died.

Hardiness is genetic. That is why some plants are hardier than
others, even why some cultivars (cultivated variety) are hardier
than others of the same plant. They may have been bred or selected
as being hardier clones.

To further confuse you, plants adapt or change genetically and
slowly over time to their climate and local habitat—something called
“provenance”. That's why a species grown in southern climates may
not be as hardy as the same species grown in a northern locale such
as Vermont. Keep this in mind when buying plants from mail order
companies or nurseries. Your best bet is a local nursery having
experience with a plant.

Roots, stems, and leaf and flower buds generally are hardy to
different temperatures. This is why many perennials die to the
ground in winter only to have their roots survive and produce new
shoots the following spring.

Forsythias, for example, often have leaves but no flowers. The
flower buds, which are less hardy than the leaf buds, are killed by
the cold. If flowers appear up to a certain height, but not above
this height, this represents the depth of snow cover that protected
the flower buds during the killing cold.

So how do you determine plant hardiness? Start by figuring out your
hardiness zone. These are geographic zones shown on maps that share
the same range of average annual minimum winter temperatures. (A few
references also will list hardiness zones for heat--the maximum
temperature a plant can endure.)

These zones are averages only. For instance, on the 2012 USDA
hardiness zone map, Burlington, Vermont is in zone 5a, which
indicates average minimum temperatures get -15 to -20 degrees F in
any year (planthardiness.ars.usda.gov). Over the last 10 years,
average lows have fallen into that range about half the time, one
year it was lower (-21 degrees), the rest were slightly warmer.

This 2012 map is more accurate than the previous 1990 map, being
based on a broader range of data (1976-2005 vs 1974-1986 for the
former), better mapping techniques, and more stations of data. Many
areas are shown as half a zone warmer on the newer map. While much
of the Champlain Valley was shown as zone 4b on the 1990 map (-20 to
-25 degrees average minimum), they’re now in zone 5a on the 2012 map
(-15 to -20 degrees).

While you mainly will find the USDA most recent hardiness zone map,
especially online, in references and in catalogs, there are some
other hardiness zone maps for the U.S. and other countries, often
with different zone numbering
(www.plantsearchonline.com/zones.htm). One is from Sunset
publications, whose zones reflect not only lowest temperatures but
other climate factors such as rainfall, latitude, and elevation.

The 2015 hardiness zone map from the National Arbor Day foundation
(www.arborday.org/media/zones.cfm) is generally similar to the USDA
2012 map, yet shows many areas in a warmer zone. Upstate New York
is mainly in zones 4 to 5 on the USDA map, but warmer zones 5 and 6
on the Arbor Day map.

When selecting plants, after looking at the hardiness zones,
consider more specific locales. Warmer zones (and colder ones) can
exist within a hardiness zone. Plants near a building may be in a
warmer zone due to heat loss from the building or the solar heat
absorbed by it. A steep southern slope may be an entire hardiness
zone or two warmer than adjacent level areas. This is called the
“microclimate” effect, and may be as small as a few square feet or a
whole landscape. Often I find a landscape may have microclimates
representing two or three hardiness zones.

In between microclimates, and the hardiness zones or
“macroclimates”, is the climate of a particular terrain or area,
such as near a body of water, a mountain slope, or a valley. This
can be termed “mesoclimate’. I like to think of microclimate as a
particular part of a property; the mesoclimate as the whole
property, neighborhood, or town; and the macroclimate or hardiness
zone as seen on maps as a whole region of a state.

When selecting plants, also you need to consider culture--conditions
such as soil type and fertility. If the soil is heavy, wet, and has
low fertility, for instance, it may stress the plant, resulting in
winter injury. On the other hand, if the soil is too fertile, the
woody plants may grow late into the season and not harden off
properly. Again, the result is winter injury. Herbaceous plants
such as perennials are different. Fertilizing late in the season
may actually make them more vigorous and better able to withstand
winter.

Another important point about herbaceous plants is that being below
ground, above ground temperatures as reflected in hardiness zone
maps are not a very accurate indicator of plant survival. There is
much latent heat in the ground that will keep these plants warmer
than the air, especially if the heat is trapped by a few inches of
snow. This is the reason some zone 6 perennials (0 to -10 degrees
F) can survive fine in zone 3 (-30 to -40) if sufficient snow cover.

Mulching can help moderate soil temperatures, similar to snow,
preventing plant injury. If you select an exposed site where
protective snow cover may blow off next winter, remember to mulch
your plants late in the fall. Even a couple inches of mulch usually
is sufficient.

Sites exposed to winter winds, especially those from the north or
west, can cause desiccation or drying out of evergreens, resulting
in leaf burn. If possible, pick a sheltered site or plan to shield
plants with a burlap screen next winter. Tree wrap or guards on
trunks will help protect trees exposed to early morning winter sun
from "frost cracking."

In summary, keep in mind these few key principles towards better
survival of your landscape plants during winter.
--Hardiness zones are only a starting point in choosing hardy
plants, and that the climate of smaller areas on your property can
influence their survival, as can variation from year to year.
--Hardiness zones are more applicable to woody plants, with parts in
winter above ground, than perennials which overwinter below ground.
These are more influenced by snow cover and mulch, both of which
trap the latent ground warmth.
--Culture affects plant vigor, which affects hardiness. Avoid plant
stresses such as too wet soils, fertility late in the season for
woody plants, and provide wind protection for evergreens.